Study of NO x from Different Natural Gas and Hydrogen Fuel Compositions in Combustion Applications

Transcription

1 Paper # 070IC-0148 Topic: Internal Combustion and Gas Turbine Engines 8 th U. S. National Combustion Meeting Organized by the Western States Section of the Combustion Institute and hosted by the University of Utah May 19-22, 2013 Study of NO x from Different Natural Gas and Hydrogen Fuel Compositions in Combustion Applications Amin Akbari, Vincent McDonell and Scott Samuelsen UCI Combustion Laboratory, University of California, Irvine, California , USA NOx emissions from stationary power generation sources such as gas turbines continue to present air quality concerns. Many studies on formation of NO x, and investigation of different formation pathways have been carried out, yet uncertainties and inconsistency in NO x formation levels in different applications is noted. This is especially an issue with alternative fuels. The literature indicates conflicting results regarding how fuel composition impacts emission levels for different applications. In an effort to address this question, NO x emissions from a lean premixed swirl stabilized combustor operated at relatively low combustion temperatures are measured in different testing conditions for different fuel compositions. The fuel composition in this study includes a wide range of hydrogen and natural gas blends, starting from pure natural gas as a conventional fuel, to pure hydrogen as an alternative fuel. Moreover, the influence of residence time on NO x emission levels is studied. To investigate the reason why emission levels change with fuel composition and residence time, a chemical reaction network (CRN) was developed. The CRN development requires detailed knowledge of flow field, thus computational fluid dynamic (CFD) simulations were conducted to facilitate the construction of the CRN. The CRN was then used to evaluate the details of the NOx formation. For the present configuration with reaction temperatures below 1800 deg-k, NNH is the dominant NOx formation pathway which corresponds to the observed increase in NOx with higher hydrogen concentrations and shorter reaction times, both of which lead to higher H radical concentrations. 1 Introduction NO x or nitrogen oxides mainly refer to nitrogen dioxide (NO 2 ), and nitric oxide (NO) which contribute to formation of ground level ozone, and small particles which both have adverse effects on respiratory systems [1]. US EPA has regulations for NO x emissions from both vehicles and stationary power plants. In addition to federal regulations, many states have more restrictive regulations for NO x emissions. Currently, the most common NO x control technology in gas turbines is lean premixed combustion [2,3]. NO x formation in lean premixed systems has been studied extensively in recent years. For instance, four NO x formation pathways have been identified as: 1) Zeldovich 2) nitrous oxide 3) Fenimore prompt, and NNH [3,4]. It is known that each pathway has specific contribution to total NO x in different combustion conditions, such as temperature, pressure, residence time, and fuel composition. It is fairy convenient to determine each pathway contribution to total NO x through chemical kinetic simulations. Each NO x formation pathway exchanges species that contribute to NO x formation in a complex manner. In addition, each NO x formation pathway depends on different parameters and can be activated in different conditions. For instance, Zeldovich NO x highly depends on the combustion temperatures. Other known parameters that affect NO x level in different pathways are residence time inside the combustion chamber, fuel compositions, fluid mechanics characteristics, flame structure, and preheat temperatures. Zeldovich NO x becomes important at temperature exceeding 1800K through following reactions: O + N 2 NO +N (1) N + O 2 NO + O (2) N + OH NO + H (3) Since the second reaction is fast, N atoms can be assumed to be in steady state conditions, and N 2, O 2, and O concentrations can be assumed to be at equilibrium. Though, this condition is valid only if there is enough time for NO formation in the first reaction which is a fairly slow reaction. The rate of NO x formation can be estimated by:

2 In this equation, k N 2 O d[ NO] dt Zeld 2k [ O][ ] (4) N N 2 O 2 is the reaction rate constant that can be found in literature, and [O] and [N 2 ] concentrations can be calculated using appropriate information about the flow field regime to choose a proper model in Chemkin. The second NO x formation pathway is nitrous oxide which forms through collisions between N 2 and O and a third body. The resulting N 2 O is attacked by O atoms to form NO. The third NO x formation pathway is prompt NO x where N 2 is attacked by CH to form HCN and N. These species are oxidized to NO within the flame in lean premixed combustion. The fourth pathway is the NNH pathway where N 2 is attacked by H atom to form NNH radical. NNH is oxidized to NO and NH. Since, in lean premixed conditions, NH is oxidized to NO as well, NNH contributes to form two NO molecules. The details of reaction rates and conditions can be found in many references [e.g., 3,4]. For natural gas fueled fully premixed lean combustion cases, NO x formation was reported to be only function of firing temperatures for temperatures less than 1900K [5]. In this work, considered as a baseline for the lean premixed combustion of natural gas, it was shown that the pressure, residence time, and preheat temperatures wouldn t impact NOx level in a well premixed lean combustion system. However, NO x formation pathways don t respond to temperature in a same manner. For instance, nitrous and thermal pathways are reported to be sensitive to temperature while prompt mechanism is a strong function of residence time [3,4,6]. Thus detailed information about different NO x formation pathways can help to identify their relative role in advanced low emissions combustion systems. For alternative fuels, the problem becomes more complicated. For instance, the impact of adding hydrogen to natural on NOx reported in the literature for atmospheric combustion conditions is inconsistent. Some report that, for a fixed adiabatic flame temperature, addition of hydrogen to natural gas doesn t impact NO x emissions [7]. Some report that NO x decreases as hydrogen is added to natural gas [8, 9]. Some researchers report that addition of hydrogen to natural gas will increase NO x emission, and some report that NO x level increases by adding hydrogen, but it becomes less severe in higher swirl numbers [10,11]. In addition, inconsistency for reported NOx emission trends exist for elevated pressure conditions as well [12,13,14,15].. Therefore, more investigations are required to explain inconsistent trend of NOx as a function of fuel composition. To investigate the impact of fuel composition on NO x emission, a chemical reaction network (CRN) concept was used. Applying CRN to a specific combustion application means to divide the reaction flow into different sub-zones with specific chemistry and fluid mechanics characteristics. To obtain details of fluid mechanics of the specific application, computational fluid dynamics (CFD) simulations can be employed. Then a reactor model is assigned to each sub zone to form a network of different reactor elements that can, when combined, provide detailed information about NO x formation in different applications. Different reactor models include PSR, PFR, and MIX. A perfectly stirred reactor (PSR) assumes an ideal reactor in which perfect mixing is achieved inside the control volume. A plug flow reactor (PFR) assumes perfect mixing in the radial direction while ignoring mixing and diffusion in the flow direction. Moreover, MIX is an element in which the entering streams are uniformly mixed without chemical reaction [4, 6, 16]. Applying one or more reactors in a network representation can result in accurate modeling of NO x formation in any special combustion process. In this study, emission of NO x from a lean premixed combustion application is studied both experimentally and numerically. NO x levels are measured for different natural gas and hydrogen compositions. The CRN is be developed using information from CFD simulations. Finally, the change in contribution of each NO x formation pathway for different testing conditions is discussed to illustrate the impact of fuel composition and residence time on NOx emissions. 2 Methods In this section the experimental setup is explained. Also, the steps of developing the CRN to study the details of emission formation are discussed. 2.1 Experimental Setup For emission measurement, a chimney is placed on top of the burner to make sampling possible. A water cooled port exists in the sampling chimney where a portion of exhaust gases is extracted to the emission measurement instrument. In this study, NO, NO 2, and CO in exhaust gas composition is measured using Horiba PG-250 which collects gas samples through a water cooled probe in conjunction with a vacuum pump. The water drop out unit collects all the moisture from the gas sample, so what is measured by Horiba PG-250 is based on dry mixture. This analyzer measures NO 2, NO, CO, CO 2, and O 2 using EPA approved methods. It utilizes a heated catalyst to convert all NO x to NO first. NO is a fairly unstable molecule which will react with O 3 to form NO 2. This reaction produces a quantity of light for each reacted NO 2

3 molecule. This light can be measured with a photomultiplier (PMT) tube or other devices. If the volume of sample gas and O 3 are carefully controlled, measured light level, which is proportional to the concentration of NO, can be interpreted as NO x (NO plus converted NO 2 ) level in the sample. In conjunction with Horiba PG-250, a Siemens N 2 O analyzer is used to measure N 2 O. The outlet of Horiba PG-250 enters the Siemens N 2 O analyzer. The gas sample leaving the Horiba PG-250 has already lost all of its moisture through the water drop unit. This configuration allows simultaneous measurement of NO x, CO and N 2 O. The schematic of emission measurement process through the explained sampling method is shown in Figure 1. Figure 1-Schematic of emission measurement To obtain accurate measurements, fuel mass flow controllers, air flows, and the output of the Horiba PG-250 are all connected to a data acquisition (DAQ) device in conjunction with Labview software. This facilitates monitoring and taking time averaged measurements of emission levels. Basically, for each data point, after maintaining the stable combustion for each case for a couple of minutes to reach the thermal equilibrium, emission levels are logged and saved for a period of time over one minute. For each specific fuel composition, air flow rate, and preheat temperature, chemical equilibrium simulations are conducted in Chemkin to check measured equivalence ratios and mass balances. After obtaining the emission level from the measurement, chemical kinetic simulation can be employed to provide detailed information about NO x formation. 2.2 Chemical Reaction Network (CRN) Development To develop and appropriate CRN, detailed information about the flowfield of the combustion application is required. CFD simulations are carried out to provide information about different aspects of reacting flow field CFD Development ANSYS 14 was used to model the combustor. First, due to the symmetric nature of the current system, a 2D axisymmetric cylindrical flow model was used to model the burner in this study. The 2D schematic with proper dimensions is drawn in Geometry sector of the ANSYS 14 package. Then, with assistance of the Mesher unit, a structural mesh is generated on the domain with around 23,000 grids as shown in Figure 2. It is noted that grid independency studies were conducted to arrive at the final mesh used. Figure 2-Mesh generated on the 2D domain. 3

4 The boundary conditions were imposed on the inlet and outlet planes and an axis was set as a centerline. Reynolds- Averaged-Navier-Stokes (RANS) equations and the 5 equation Reynolds Stress Model (RSM) turbulence model were used for the simulation. The RSM model was chosen over k-epsilon because (1) of the recirculating flow in the combustor and the strong pressure gradient fields in the high swirling flow and (2) it is a less dissipative turbulence model. Higher order discretization schemes are used. For pressure, the PRESTO spatial discretization mode was used. For momentum, the QUICK discretization was used. To model the swirling effect, as the swirl number was calculated and tangential and axial velocity ratios are adjusted in the inlet boundary condition. To add reaction to the flow, a skeletal mechanism was incorporated within Fluent as the chemistry set for modeling chemistry in CFD simulations [17]. For species and energy, a third-order MUSCL Scheme was used. The turbulent combustion within the reactor is computed using the Eddy-Dissipation-Concept (EDC). The Discrete Ordinates (D-O) model accounts for the radiation from gases inside the combustor. An example of temperature contour for the case of 1800 adiabatic flame temperature for the mixture of 50/50 H 2 /NG with 500K preheat temperature is shown in Figure 3. Figure 3- Temperature Contour for 1800 AFT with 500 Tp for 50/50 H2/NG After obtaining reliable CFD simulations, the CRN can be developed. Good prediction is reported for the flame region using a PSB followed by a PSR with the volume of the flame [18]. Isopleths of CO ppm are used to indicate the extent of the reaction zone in CFD. The reason is that the highest concentration of CO is present in the flame zone as an intermediate radical. The same result for flame volume is obtained using reaction rate of CH 4 and CO and other radicals. The contour of CO ppm is shown in Figure 4. Figure 4-Contour of CO ppm (indication of flame front) for 1800 AFT with 500 T p for 50/50 H 2 /NG. 4

5 The boundary of the flame is defined as 0.3 of the maximum CO concentration, which is consistent with criteria used previously [e.g., 18]. Using this criterion, the flame volume can be calculated. For instance, for the conditions shown in Figure 4, flame volume was calculated to be 62 cm 3. This number will differ for other flame temperatures and fuel compositions. The other information needed for developing the CRN is the percent of flow that is recirculated. This can be obtained from velocity and densities over cross sections through the recirculation zones. The boundary of the recirculation zone is similar to solid walls since no net flow crosses it. Inside the recirculation zone, the axial velocity profile starts from zero at the axis wall (Y=0), drops below zero within the recirculation zone and then starts increasing again till it reaches the upper boundary of the recirculation zone. After that the axial velocity is positive up to the upper wall. For described conditions, 1800 AFT with 500 Tp for 50/50 H2/NG, the recirculated flow was about 18% of overall flow. In addition to flame volume and percent of recirculated mass, the velocities across the domain can also be used to estimate residence times for other elements in the CRN CRN design In this study, two types of CRNs were tried to model the chemical reaction within the reaction. The first type consisted of only two elements, a PSB and a PFR. For each mixture and conditions, the PSB was established by running chemical kinetic simulation for a PSR up to the blowoff limit. The configuration is shown in Figure 5. The green dashed line represents the recirculated mass flow rate. The element after the PFR is a splitter which allows the flow exiting the PFR to be divided. As mentioned above for one described conditions, the split is 18 percent recirculated gas and 82 percent to the exhaust. Although it was calculated for specific conditions, CFD results for other cases showed very similar results, so 18 percent recirculation was used for most cases. Figure 5-CRN I with PSB and PFR The benefit of using CRN I showed in Figure 5 is that it is very simple and easy to simulate. It can provide quick simulation results for the cases. However, it doesn t take account for the flame zone where fast reactions are occurring. To include this effect, CRN II was developed as showed in Figure 6. The volume for PSR element which represents the flame was calculated with a procedure explained above. 5

6 Figure 6-CRN II with a PSB, PSR and PFR Comparison of measured and predicted results indicated that CRN I overpredict the NO x levels in high hydrogen content cases, so CRN II was applied for most of the cases in this study. With the CRN developed, emission levels for all the cases are considered. For this study, levels of CO, NO, and N 2 O are evaluated. Measured values of NO were compared to modeled NO from CRN. For each case, three additional simulations are applied to evaluate the relative contribution of each pathway. The strategy is to calculate the total NO first. Then all the rate limiting reactions for thermal NO were disabled. The difference between the total NO and second simulation will be thermal NO. Next, rate limiting reactions of thermal and N 2 O pathways are disabled. Comparing the third and second simulations yields the N 2 O pathway contribution. Last, rate limiting reactions of thermal, N 2 O and NNH pathways are disabled. Comparing the result of the fourth simulation and the last three simulations, NO formed via the NNH pathway can be calculated. Also, by looking at the simulation results of fourth reaction, prompt NO is achieved. For all the cases, experimental measurements of CO, NO and for some cases N 2 O are used to validate the CRN simulations. Then analysis of the NO formation pathways can be accomplished. 3 Results and Discussion In this section, experimental measurements and CRN modeling of NO x from combustion of different conditions is discussed. Fuel compositions ranging from pure natural gas to pure hydrogen are studied. Different adiabatic flame temperatures and residence times are considered for testing. For most cases measured CO and NO x levels are used to verify the developed CRN II as explained before, and then different NO x formation pathways are calculated using chemical kinetic simulations in Chemkin Pro. The impact of residence time and fuel composition on NO x levels at 1800K AFT is presented first, followed by results for 1900K AFT. Remarks regarding NO x formation pathways are then presented K Adiabatic Flame Temperature In this section the NO x results for 1800K AFT are discussed. It should be noted that to keep AFT constant, a reaction equilibrium simulation is conducted to calculate appropriate equivalence ratios for combustion. Then natural gas and hydrogen flow rates are adjusted in LabView to deliver that equivalence ratio for the given air flow rates. For all the cases in this section, an air preheat of 500K is used. Air flow rates are also fixed when studying the effect of fuel composition on NO x levels. However, to study the impact of residence time, air flow rates are changed to accommodate different flow residence times inside the combustor. In the case of fuel composition, eight fuel compositions are studied including: 100/0, 80/20, 60/40, 50/50, 40/60/ 20/80, 10/90 and 0/100 in NG/H 2 blend. For this study, the air flow rate is 23.6 g/s which corresponds to a 20 ms 6

7 residence time. For the case of residence time, two different fuel compositions are chosen to represent the impact of residence time on NO x levels. First one is the pure natural gas case, and the second one is 50/50 NG/H 2 case. The impact of fuel composition on NO x levels is shown in Figure 7. Figure 7-Impact of fuel composition in NG/H 2 blend on NO x, 1800K AFT, 500K T p, 20ms Residence time. As shown, the comparison of CRN predictions and measured NOx levels is very good. Both CRN and measured values suggest a modest increase in NO x levels from pure natural gas up to 50/50 NG/H 2 blend. Then NO x levels are relatively constant until pure hydrogen where there is a slight decrease in NO x. Figure 7 also illustrates that prompt NO x decreases as H 2 percent in NG/H 2 blend increases. This is due to the concomitant decrease in CH with decreasing hydrocarbon in the fuel mixture. It is also noted that NNH is the dominant NO x formation mechanism in this set of cases. Note that NNH NO x constantly increases as hydrogen in fuel increases. This observation is consistent with findings of other researchers that NNH is a major contributor in NO x levels in NG/H 2 mixtures [19, 20, 21, 22, 23]. As hydrogen increases, H atom, the key element in NNH pathway, increases, and NO x goes up. Figure 7 also illustrates that NOx from the Zeldovich pathway remains relatively constant for al fuel compositions. NOx from the N 2 O pathway, although expected to increase as hydrogen percent goes up, actually decreases. There could be two reasons for this trend, first is that as hydrogen, which is a smaller molecule than natural gas, increases the number of large intermediate species in the chemical mechanism, decreases. It is noted that nitrous (N 2 O) NO x formation requires a collision between O atom and N 2 and a third body (M). As concentration of M in the mixture decreases, nitrous NO x level could be affected. Also, as NNH pathway contribution increases, available O atom might be consumed in NNH+O reaction to form NO x. That might affect the availability of O atom which is necessary in forming NO x via the N 2 O pathway. The role of residence time on CO and NO x levels is illustrated in Figure 8. The CRN analysis predicts that CO levels decrease with an increase in residence time. This is due to the fact that, with more time, O atom concentration decreases, and less CO is formed. Also, CO has more time to react with oxygen to form CO 2. In terms of NOx, as residence time increases NOx decreases modestly via all four pathways. For Zeldovich it could be due to the fact that the Zeldovich NO x here is due to reaction of super equilibrium O with nitrogen. As residence time increases, super equilibrium O decreases. For N 2 O pathway NO x, decay of O atom as time increases could be the reason for decrease. For NNH and prompt NO x with more residence time there is more time for radicals to decay, so both decrease with increasing the residence time. To study the impact of residence time on hydrogen content fuels, a 50/50 NG/H 2 mixture was chosen. The CRN and measured results for NO x and CO are shown in Figure 9. Figure 9 shows similar results as for natural gas shown in Figure 8 with NO x and CO decreasing with increased residence time. 7

8 Figure 8-CRN and Measured NO x and CO, 1800K AFT, 500K T p, natural gas. Figure 9- CRN and Measured NO x and CO, 1800K AFT, 500K T p, 50/50 NG/H K Adiabatic Flame Temperature In this section, NO x levels as a function of fuel composition and residence time are evaluated for fixed 1900K AFT. By comparing these results with those above, the influence of AFT on the different NO x formation pathways can be assessed. The impact of fuel composition on NO x is shown in Figure 10 which again illustrates good agreement between CRN and measured values. Starting from pure natural gas, it is observed that, by adding hydrogen up to 80%, NO x increases. NO x then decreases with 90 and 100 percent hydrogen in the fuel. Interestingly, the NO x level in pure hydrogen and pure natural gas is comparable. Another interesting point is that, for the case of 1800K, the slight decrease in NO x by adding hydrogen occurred after 10/90 NG/H 2 mixture, but for the 1900K this decrease happened after 20/80 NG/H 2 mixture. It is clear that NNH NO x is again the dominant pathway in this application. Although the total NO x decreases after 20/80 NG/H 2 mixture, NNH continues to increase. This could be due to very high concentration of H atom in the reaction where the concentration of hydrogen is very high in the fuel. N 2 O and prompt NO x decrease rapidly after 20/80 NG/H 2 mixture. For prompt it is due to lack of hydrocarbon radical in the mixture. However, for N 2 O NO x it is less clear what the reason is. Again, it could be due to absence of third body collision in those cases, or lack of O atom which is consumed in the dominant NNH pathway. Anyhow, it is what can be seen with both experimental results and CRN predictions. The study of NO x versus residence time provided similar trends as those for 1800K AFT for both natural gas and 50/50 NG/H 2 mixtures. Further study showed that contribution of each pathway changes with AFT, so that after certain AFT, Zeldovich mechanism becomes the dominant one. Thus, different emission behavior is expected. 8

9 Figure 10- Impact of fuel composition in NG/H 2 blend on NO x, 1900K AFT, 500K T p, 20ms Residence time 4 Conclusions Impact of alternative fuels on NO x emission is complicated and needs broad investigation. The Chemical Reaction Network (CRN) approach is a very strong tool to provide information about details of emission formation in combustion application. Contributions of different NO x formation pathways to the total NO x depend on the fuel composition and testing conditions. In the present work, the NNH formation pathway was dominant for almost all fuel compositions, residence times, and 1800K and 1900K adiabatic flame temperatures. In general NO x goes up with parameters that lead to an increase in NNH formation pathway which are increase in H atom in the reaction. This occurs with increase of hydrogen concentration in the fuel and reduction in residence time to avoid decaying H radicals. Identifying major NO x formation pathways can result in better understanding of why NO x levels change with variables such as adiabatic flame temperature, fuel composition, preheating temperatures, residence times, and etc. Acknowledgements The authors acknowledge the support of the California Energy Commission (Contract : Marla Mueller Contract Monitor). The assistance of Brendan Shaffer and Adrian Narvaez in the setup of the experiment and data acquisition systems is greatly appreciated. The help of Roberto Fonesca and Steven Lee regarding the data acquisition systems is also noted. Discussions with Megan Karalus and Igor Novosselov from the University of Washington regarding the CRN approach are gratefully acknowledged. References [1] [2] A. H. Lefebvre and D. R. Ballal, 2010, Gas turbine combustion: alternative, fuels and emissions. Taylor & Francis: Boca Raton. [3] T. Rutar, P.C. Malte, 2001, NOx Formation in High-Pressure Jet-Stirred Reactors With Significance to Lean- Premixed Combustion Turbines, Proceedings of ASME: TURBO EXPO 2001 [4] Turns, Stephen R. An introduction to combustion. Vol New York: McGraw-hill, [5] G. Leonard, J. Stegmaier, 1994, Development of an aeroderivative gas turbine dry low emissions combustion system, Journal of engineering for gas turbines and volume: 116 issue: 3 page:

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